In manufacturing thermoplastic resin compositions, it is well known that free, unpolymerized monomer often remains absorbed within the thermoplastic resin produced due to the fact that complete monomer polymerization (particularly monomers such as styrene) is impractical to achieve on an industrial scale. Additionally present may be minor amounts of other organic species such as solvents, oligomers, catalyst, or volatile condensation products. Such substances may be detrimental to the ultimate product formed from the thermoplastic resin by reason of off-taste, off-odor, toxicity, or degradation of physical properties via plasticization, depolymerization, and so forth. Additionally, it is expected that government regulatory agencies may eventually establish maximum permissible levels of various monomers, including styrene, in packaging materials intended to contact food, beverages, pharmaceuticals, and cosmetics on the ground that excess levels represent an unacceptable health risk.
U.S. Pat. Nos. 4,940,472 (Hay et al.) and 4,952,672 (Moore et al.) disclose centrifugal devolatilizers which remove volatiles such as unreacted monomer from a variety of polymer products. The patents suggest the use of carbon dioxide added to the polymer to provide a stripping action, but fail to describe how the carbon dioxide could be effectively introduced. Moreover, such centrifugal devolatilizers are not in widespread commercial use; utilization of such equipment to remove monomeric residues from thermoplastic resins would require substantial capital investment by the plastics industry.
U.S. Pat. No. 4,703,105 (Allada) discloses the extraction of residues from molten styrenic polymers using a solvent gas such as carbon dioxide at near critical to supercritical conditions. According to the patent, the process may be conducted by bubbling or sparging the gas through the polymer contained in an extruder. Uniform, rapid diffusion of a gas throughout a molten polymer is required into order to maximize the efficiency of the residue extraction, yet is quite difficult to accomplish in the absence of highly sophisticated and expensive processing equipment.
U.S. Pat. No. 5,204,410 (Banevicius et al.) teaches that volatile substances in polyphenylene ether/polystyrene compositions are substantially reduced by extruding the resins in a single pass using a plurality of stages comprising injection of a stripping agent (preferably water) and vacuum venting. While the patent suggests that carbon dioxide could be used as the stripping agent instead of water, it does not indicate how a substance such as CO.sub.2, which is much more volatile than water, could successfully be introduced into such compositions to efficiently remove the impurities present.
Thus, there exists a great need to develop an effective technique for reducing the level of volatile impurities in thermoplastic resins which is readily capable of adaptation to existing resin processing equipment.